284 PASTEUR : THE HISTORY OF A MIND 



however, a development of the microbe, since there 

 has been disease, and, in reahty, when this disease is 

 in progress, the organism can be found at the point of 

 inoculation and in all the tissues, but this time it has 

 not caused death. The chicken has repelled the attack. 



Has, then, this benign cholera played in the chicken 

 the role of the benign smallpox or vaccine in man? Yes, 

 for this chicken is henceforth immune to inoculation with 

 the youngest and most virulent culture of the micrococcus. 

 It has been vaccinated against the cholera. 



Let us continue this study which has already proved 

 so fruitful. Since the power of action on the organism 

 diminishes with the age of the culture, let us make 

 our inoculation from a very old culture, one which is 

 near the point of death. The microbe, still alive, can 

 still cloud the bouillon into which it is sown. It does 

 this slowly, but life and virulence are not synony- 

 mous in this case, for the organism is absolutely incapable 

 of developing in the body of a chicken, or communi- 

 cating to it even the slightest disease. Here it is the 

 chicken which overcomes and kills the inoculated microbe 

 straightway. The latter, in spite of its specific origin, 

 entirely resembles, therefore, those thousands of spe- 

 cies of micrococcus which are met with everywhere on 

 the surface of the body, which fill the intestinal canal, 

 and which are always harmless. It is, nevertheless, 

 that same microbe which some weeks before killed ten 

 chickens out of ten. A virus, then, is not that entity, 

 not that unity, which it was considered to be by the 

 old physicians. It is in a state of perpetual evolution, 

 of continuous variation, due to wholly natural causes. 



Let us take, now, this last chicken which suffered no 

 inconvenience from the inoculation with our dying 

 virus, and let us try to inoculate it with the virulent 

 virus; it behaves toward this exactly as a new chicken 



